1. To have or suffer a continuous, dull pain
2. To feel great sympathy, pity, or the like
3. To feel eager; yearn; long
1. A continuous, dull pain
|Me, for the last three days (source)|
The spelling differences between the noun and verb forms reflected a difference in pronunciation. The noun was pronounced like modern ache, but the verb form had a soft \a\ and the 'ch' sounded like it does in watch ([ɑtʃ] if you know IPA). By the 1700's the two forms had the same pronounciation, and the spelling of both changed to ache based on a false assumption that their origin was Greek akhos ("pain, distress").